The first run at it was pretty good but couldn't handle all of the variations in financing and didn't have a user-friendly printed output.  

Landlords are so short-sighted when it comes to hiring property management companies and always mention some variation of your, "but 8% seems a lot..." nearsightedness.How much do you know about the following:1) Fair Housing laws2) Fair Credit Reporting Act3) Consumer Privacy Protection laws4) Local rental ordinances5) State eviction laws and procedures6) Etc...Everyone thinks property management is easy until something goes wrong...

@Nathaniel WalkerIt’s really hard to explain because honestly there is a lot of variation.

Once you add in the variations and the subtle nuances, you may literally have hundreds if not thousands of different trusts. 

I am assuming the zoning variation was for a detached garage.  

For example, that other calculator bases maintenance costs on a percentage of property value (whereas obviously the 50% rule does it as a function of rent).I'm also looking in a couple different areas where the tax rates have an insane amount of variation (I'm talking a 10x or more difference).

But, there are a host of other factors not as obvious to the borrower including if it a single family or a multi-family, when you are looking to close (the sooner the better in rate terms), how many points  you are willing to pay, for the dscr if you have experience as an investor or not, and right now there is a LOT more variation between lenders then usual due to the volatile rate environment we are in right now.

Setup some tests on your own site and test some different variations and then go w/ what performs the best.  

Our best deals have always come from rentals using the Slow Flip Strategy, a variation of the BRRRR strategy.The first and second deal we did using that strategy had a gross profit of $100,560 and $119,970.

We are now at the point where he states I need new pre-approvals due to a slight variation in my variable commission.